Progesterone and Prolactin: Hormones Important for the Reactivation of Toxocara canis Larvae in Bitches

Traditionally, prolactin has been the only hormone associated with the reactivation of arrested Toxocara canis larvae in pregnant bitches. Nevertheless, recent findings have shown the effect of progesterone on the host immune response and on the physiology and behavior of the parasite, which probably causes the reactivation of arrested larvae in pregnant bitches. The initial presence of T. canis larvae in the organs of the host causes a non-specific inflammatory infiltrate to surround the larvae, after which a Th1-type response is induced, which is characterized by the recruitment of macrophages and leukocytes, and the production of Interferon-gamma (IFNγ) and pro-inflammatory cytokines that participate in granuloma formation. In parallel, a Th2-type response is mounted, which regulates the severity of the granulomatous infiltrate that surrounds the larvae, helps in larvae survival and likely favors larval arrest. Larvae in pregnant bitches are reactivated in the second part of gestation, which is characterized by an initial increase in progesterone followed by an increase in prolactin. Progesterone modulates the immune response and induces an increase in anti-inflammatory cytokines, especially interleukin (IL)10, which decreases the severity of the granulomatous response, creating favorable conditions for larval arrest. Furthermore, progesterone and prolactin stimulate the growth and development of larvae in vitro through action on possible receptors for these hormones. In conclusion, there is wide evidence of a strong association between both hormones and T. canis larvae reactivation in the second part of gestation in bitches.